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Science and Technology of Future
Light Sources

A White Paper

Report prepared by scientists from ANL, BNL, LBNL and SLAC. The coordinating team
consisted of Uwe Bergmann, John Corlett, Steve Dierker, Roger Falcone, John Galayda, Murray
Gibson, Jerry Hastings, Bob Hettel, John Hill, Zahid Hussain, Chi-Chang Kao, Janos Kirz,
Gabrielle Long, Bill McCurdy, Tor Raubenheimer, Fernando Sannibale, John Seeman, Z.-X.
Shen, Gopal Shenoy, Bob Schoenlein, Qun Shen, Brian Stephenson, Joachim Stöhr, and
Alexander Zholents. Other contributors are listed at the end of the document.

Argonne National Laboratory

Brookhaven National Laboratory

Lawrence Berkeley National Laboratory

SLAC National Accelerator Laboratory

December 2008

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Argonne National Laboratory
9700 Cass Avenue
Argonne, IL 60439

Brookhaven National Laboratory
P.O. Box 5000
Upton, NY 11973-5000

Lawrence Berkeley National Laboratory
University of California
Berkeley, CA 94720

SLAC National Accelerator Laboratory
2575 Sand Hill Road
Menlo Park, CA 94025


Arthur L. Robinson (LBNL) and Brad Plummer (SLAC)

Illustrations and layout by:

Terry Anderson, Gregory Stewart, Sharon West, InfoMedia Solutions (SLAC)


This document was prepared as an account of work sponsored by the United States Government. While this

document is believed to contain correct information, neither the United States Government nor any agency thereof,

nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty,

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s

use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use

would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service

by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement,

recommendation, or favoring by the United States Government or any agency thereof or its contractors or

subcontractors. The views and opinions of authors expressed herein do not necessarily state or reflect those of the

United States Government or any agency thereof or its contractors or subcontractors.

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S.

Department of Energy under Contracts No. DE-AC02-06CH11357 (ANL), DE-AC02-98CH10886 (BNL), DE-

AC02-05CH11231 (LBNL) and DE-AC02-76SF00515 (SLAC).

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Science and Technology
of Future Light Sources  


metal and impulsively flip the sign of the exchange interactions? How would such an exotic state of matter
look like?

Other areas of interest may result from multi-photon spectroscopies, extending the NEXAFS technique to
multi-photon excitations. In this way, one could access different selection rules for the NEXAFS lines, and
understand the hybridization of solids in new ways. Pairs of FEL pulses could be used, for example, to
study p and d character of a bond by studying linear and two-photon absorption. Other extensions of
nonlinear optics could be used to study complex electronic properties, from hole-burning spectroscopy in
the x-ray range to multi-dimensional studies of ultrafast electron dynamics.

THz Excitation

The far-infrared or terahertz spectral range consists of photons with meV energies that drive low frequency
vibrations and intra-band electronic transitions, including collective phenomena such as charge density
waves, excitons and superconductor pair-breaking. To date, nearly all experiments that study the ultrafast
response of solids have been performed with visible or near visible excitation. Yet one often wishes to
understand how a dynamically fluctuating ground state, perturbed on thermal energy scales, couples to the
electronic structure. Most of the excitations that do affect the physics of these compounds occur at
energies near or below room temperature, i.e. on energy scales well below 100 meV corresponding to the
THz spectral range. While THz laser systems have been available for decades, few have been capable of
producing ultra-fast, single cycle pulses with intensities sufficient to induce measurable behaviors. As
such, we are only beginning to explore the effects of high power THz fields on matter, as it has only
recently become possible to produce ultra-strong half-cycle pulses of THz radiation with electric-field
strengths rivaling bonding fields in materials (~ V/atom) and magnetic fields exceeding typical laboratory
fields (> 10 Tesla). Having access to such strong transient electric and magnetic fields, one can consider
addressing some important questions such as “Can one switch, or modulate, the collective magnetic state
of a complex oxide at THz frequencies? Can one photo-induce a quantum coherent state and, indeed,
superconductivity? Can one control magnetic systems (and frustration) on the ultrafast timescale?

First experiments, conducted by exposing ferromagnetic thin films to strong transient magnetic fields,
have revealed remarkable new physics that is presently poorly understood. The ultra-strong fields are
found to lead to a nonlinear magnetic response that fractures the magnetization and imposes a speed limit
on the technologically important magnetic-switching process. The electric fields are capable of distorting
the atomic valence charge on the level of ligand field-like anisotropies and lead to a transient novel
magneto-electronic anisotropy, possibly opening the door to manipulating the magnetization by electric
fields alone. Most surprising is the absence of Joule heating in the sample. This indicates that extreme
electric fields can temporarily transform a transition metal into a new electronic state, indicating physics
beyond present models. These experiments are just the first experimental evidence for unusual behavior of
matter in extreme THz fields. An entire field is open for exploration and, in particular, the combination of
THz pump and x-ray probe experiments will offer the exciting opportunity to separately study the atomic,
electronic and magnetic response of materials to extreme electromagnetic fields. A few specific
opportunities are described below.

Recently, it was shown by the Cavalleri group that 15 THz radiation could be used to directly excite a
single IR-active vibration in various manganites and drive an ultrafast electronic, orbital transition. Such
vibrationally driven transitions can be excited with a small fraction of the energy (<1%) necessary to heat
the sample to the transition temperature, directly modulating the distance of some selected bonds. Such

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Examples of Scientific Drivers for Future X-Ray Sources


experiments are then controlling the bandwidth of the solid on the ultrafast timescale. In a sense, these
experiments are the ac equivalent of high-pressure studies. Yet, in these first phonon-induced phase
transition experiments, only the mid-IR region near 17 THz (17 m) could be accessed, and only high
frequency modes that stretched oxygen and metal bonds could be driven. In perovskite structures one
would like to understand how stretching (15 THz), but also bending (10 THz) and external (5 THz) modes
couple to the electronic structure, when excited coherently at large amplitudes, could be coupled to the
electronic structure. In other systems, one is interested in selectively exciting superconducting gaps in the
few THz range. Other collective excitations, like charge and spin fluctuations, or even Josephson plasma
resonances could be driven at high amplitudes into the non-linear regime, and understand excitation,
coupling to other modes. New approaches for the control of quantum coherence may result from these

Probing at x-ray wavelengths would enable measurements of the photo-induced pathways by tracking (a)
the amplitude of the light induced distortions with hard-rays, (b) the rearrangements in orbital, spin and
charge order with soft x-ray scattering and spectroscopy, (c) the rearrangement in the Fermi surface with
time and angle-resolved photo-emission spectroscopy, (d) the evolution of domains by coherent imaging
with femtosecond resolution.

One often encounters novel materials physics in the vicinity of phase boundaries. Materials such as
ferroelectrics and multiferroics show spontaneous polarization (electric, magnetic and combination) below
a critical temperature, sometimes associated with a structural distortion. A strong THz pulse could be used
to drive the material through the relevant transition while the structure is probed by x-rays to follow the
dynamical evolution of any distortion (Figure 3.19). The picosecond time scale is particularly relevant
since these distortions are usually evident as phonon mode softening into the THz spectral region. For
applications that depend on switching of the ferroelectric or ferromagnetic state, the device speed is often
limited by domain wall motion. Intense THz pump / x-ray probe studies may allow the fundamental limits
for wall motion to be quantified.

Finally, we note that the electrical infrastructure for meeting the Nation’s needs in transporting energy
efficiently and reliably depends on high-strength dielectric materials and potentially (in the future)
superconducting materials. The strong electric field of a THz pulse could be used to trigger a breakdown
event and allow the initial process dynamics to be probed on an ultra-fast time scale. UV and soft x-ray
spectroscopies would be useful probes of the valence and core level electronic structure and occupation as
the breakdown process developed and evolved. Similarly, a strong transient E-field applied to a
superconducting material will drive the flow of supercurrents and eventually disrupt the superconducting
state. The dynamics of this process are unknown, but ultimately it should lead to a collapse of the gap-
structure in the spectrum of electronic excitations and the loss of long-range phase order. If the process
occurs on a time scale short compared to thermalization, disrupting superconductivity with a strong THz
pulse may enable the study of material properties that are otherwise masked by the superconducting phase.

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Interactions of X-Rays with Matter: Perturbative Limits and Mitigation Strategies


then see the modified electronic state and at delay times of picoseconds it would probe the atomically
excited sample, which may even have been destroyed.

Another effect may prevent physical “damage” of a sample when the illuminated sample volume becomes
very small, e.g. for beams with lateral sizes smaller than a micrometer. In our earlier discussion of
“damage”, we have considered only the energy deposited in the sample volume illuminated by the beam
but not the heat flow out of the volume. In general, this assumption overestimates “damage” effects.
Furthermore, we have used the simple metric of constant fluence or energy per unit area to determine
damage limits. As the area decreases, this metric leads to a drastic reduction in the tolerable number of
incident photons per pulse. In practice, in this way we may vastly overestimate damage effects. We know
that electron transport in macroscopic structures like wires is limited to certain current densities because of
heating and the ultimate melting of the wire. If the macroscopic rules were applied to nanowires, much of
our present technology would not be possible. For example, a macroscopic Cu wire of 1-mm diameter has
a current density survival limit of about 10


. In metallic Cu nano-pillars of ~100-nm diameter one

can achieve current densities 10

without destroying the device. This increase of five orders of

magnitude is due to the significantly increased surface-to-volume ratio and the resulting high cooling rate
through the surface of the nano-region. We expect similar “cooling” effects when the x-ray beam size is
reduced, and hence the physical damage limit would be significantly increased. We note, however, that
such thermal equilibration proceeds on the slower pico- to nanosecond time scales.

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This document was coordinated by, and includes written contributions from, the team listed at the
front of the document.

We acknowledge written contributions from:

John Arthur (SLAC)
Hendrik Bluhm (LBNL)
Axel Brunger (Stanford)
Larry Carr (BNL)
Andrea Cavalleri (Univ. of Hamburg, Germany)
Eric Colby (SLAC)
Tom Devereaux (Stanford/SLAC)
Kelly Gaffney (Stanford/SLAC)
Oliver Gessner (LBNL)
Janos Hajdu (Uppsala)
Britt Hedman (Stanford/SLAC)
Russel Hemley (Geophysical Laboratory)
Alexander Hexemer (LBNL)
Keith Hodgson (Stanford/SLAC)
Georg Hoffstaetter (Cornell)
Raymond Jeanloz (Berkeley)
Kwang-Je Kim (ANL)
Richard Lee (LLNL)
Wim Leemans (LBNL)
Aaron Lindenberg (Stanford/SLAC)
Wendy Mao (Stanford/SLAC)
James Misewich (BNL)
James Murphy (BNL)
Anders Nilsson (Stanford/SLAC)
Claudio Pellegrini (UCLA)
James Safranek (SLAC)
Peter Siddons (BNL)
Hans Siegmann (SLAC)
Christoph Steier (LBNL)
Hiro Tsuruta (SLAC)
Vittal Yachandra (LBNL)
Bill Weiss (Stanford/SLAC)

We acknowledge helpful comments from:

Paul Adams (LBNL)
Michael Borland (ANL)
Joe Bisognano (SRC)
Phil Bucksbaum (SLAC)
Robert Byer (Stanford)
Yunhai Cai (SLAC)
Swapan Chattopadhyay (Cockroft Institute, UK)
Jeff Corbett (SLAC)
Winfried Decking (DESY, Germany)
Peter Denes (LBNL)
Seb Doniach (Stanford)
Pascal Elleaume (ESRF, France)
William Fawley (LBNL)
Ben Feinberg (LBNL)
John Fox (SLAC)
Miguel Furman (LBNL)
Efim Gluskin (ANL)
Rod Gerig (ANL)
M Zahid Hasan (Princeton)
Toshio Kasuga (KEK, Japan)
Steve Kevan (Univ. Oregon)
Sam Krinsky (BNL)
Steve Leone (Berkeley/LBNL)
Donghui Lu (SLAC)
David Moncton (MIT)
Howard Padmore (LBNL)
Hirohito Ogasawara (SLAC)
Dave Robin (LBNL)
Eli Rotenberg (LBNL)
Ronald Ruth (SLAC)
Andy Sessler (LBNL)
Sami Tantawi (SLAC)
Hans Weise (DESY)
Herman Winick (SLAC)
Mike Zisman (LBNL)

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